1. VIRTIS flyby of Steins M-IR Spectral analysis
Virtis Rosetta TM, Paris 10-12/05/2010 
Stéphane Erard & Virtis-H team

2. Virtis Steins geometry
Version presented at the previous TM (computed February 2009)
Uses:
• Refined S/C attitude kernel from Osiris images (Oct 2008)
• PC6 pointing info for Virtis H
• Steins figure from early Osiris estimates (ellipsoid 2.87 x 2.45 x 2.30 km)
Next version, May/October 2009:
Osiris plate model implemented in our geometric code
• Then using the Spice development routines for plate models (DSK)
Uses L Jorda model with L Kamp spice routines & F Tosi advices,
interfaced with GeoRos by X Bonin
+ some improvements in GeoRos (H/M coregistration better than 1 px)

3. M data projection Disk orientation as seen from S/C
3D animation :
I1_ wrl
Before start of acquisition
At end of acquisition
Rotation during
acquisition ~40°
Osiris 18:38:00
Osiris 18:39:18
See animation for acquisition scheme:
PI895I31_2a.gif

4. M cube projections Angle values are fully consistent with the data
OK within ~1 pixels, with reduced accuracy at the limb
Correspondence between observed and retrieved craters.
Limitations are assumed to be related to model orientation
Estimated intercepts:
From plate model
From spheroid
(previous results)
Incidence angle
Emergence angle
Measured I/F
From DTM
Measured I/F

5. 1- M vs H features Example M spectrum
M-IR contains a systematic “bump” at 4.87 µm
2 absorption bands on the sides?
Not a calibration effect
4.87 µm Band depth       Reflectance

6. M vs H features H status:
• Steins acquisition perturbed by on-board software (fixed for ESB3)
=> oscillations at short wavelengths
• FOV generally not filled by the target
=> grating not entirely illuminated
Early studies : no 4.87 µm feature detected (except a single cold pixel)

7. M vs H features H acquisition:
From FOV projection on plate model: 22 spectra with H FOV entirely filled
=> can be used to study detailed spectral features

8. M vs H features
H: 22 pixels with FOV filled by Steins - preliminary calibration
M: 104 pixels far from limb
H synchro issue
M filter defect

9. M vs H features H preliminary calibration:
No feature detected from these 22 spectra
H TF to be improved

10. M vs H features No discrepancy on ESB3 (Earth night side)
=> No systematic calibration issue

11. M vs H features Conclusions:
• H channel does not confirm the 4.88 µm feature from M channel
• H transfer function still to be improved, but TF is unlikely to conceal an existing feature
H TF to be improved

12. 2- Spectral modeling (updated)
• Simultaneous inversion for T and spectral reflectance
• Requires a photometric model to relate reflectance and emissivity
• Results based on Lambertian model
L-S provides similar results with slightly lower T

13. Spectral inversion: using plate model geometry
Provides realistic T and reflectance
T is slightly lower than with basic geometry
4.8 µm feature always appears as a peak in reflectance (emitted contribution is smaller)
whatever the model used, and for every pixel
Upper twist at long wavelengths => mix of temperatures?
=> T is too low?

14. Inversion: Temperature map
Pixels near the limb may not be entirely filled
From plate model
Reflectance     Temperature map   Temperature map
     (limb excluded)     (all pixels included)
Temperatures histogram

15. Temperature variations
Lambertian surface: T  cos(i)1/4
r  cos(i)
Good correlation (>0.8) observed far from limbs
Temperatures histogram
limb pixels in blue

16. Inversion: average spectrum & dispersion
Error bar = ±5 s
• 4.8 µm feature is a peak in reflectance (emitted contribution is smaller)
• Mix of temperatures? (either lateral or sub-surface gradient)

17. Analysis of variations
Reflectance analysis based on ICA, limb excluded — main 3 components
1st comp (33% contrib)
2nd comp (23% contrib)
5th comp (21% contrib)
Three main sources of variation:
signal magnitude
filter crossings / hot pixels + low frequency variations / slope near limbs
filter pattern around 3 µm
Reflectance 1 µm

18. Spectral inversion: fitting upper envelop
The 4.87 µm dip is expected to occur between 2 bands
=> thermal emission should be larger
With surface T ~ 6 K higher
• Suppresses upward twist
• “Band” profile does not look like a natural absorption band?

19. Conclusions Geometry is now OK
• Target intercept is OK within ~1 M-px
• Correspondence between measured signal and retrieved geometry is not perfect,
though (craters - DSK version?)
=> Geometry files are available on Otarie server
No confirmation of the 4.87 µm M-feature by Virtis-H
• Calibration is being updated, but no feature is expected
Spectral / Temperature inversion on M-IR
• Temperature map + reflectance spectrum & variability
Slight improvement relative to ellipsoid projection
• On M-IR, one single feature observed: the bump at 4.87 µm
(previously suspected features are artifact: 3.46 µm is hot pixel, 4.32 µm is a filter)
• Surface follows the Lambertian model in good approximation